MVC Blues - Who's In Control Here Anyway?

OK, this has me stumped...I have a Posts controller and model and a
Comments controller and model. Posts has a show method and a
corresponding view that shows a post and its associated comments, which
are rendered with a partial
from comments (<%= render :partial => "comments/comment", :collection =>
@post.comments %>). Another partial from comments is used to allow the
to add their comment to the page (<%= render :partial =>

Problem: If a user tries to add a new comment but it fails the comment
model validation, how do I get back to the show page from posts and
display the model validation error for the new comment?

def create
  @comment =[:comment])
    redirect_to :controller => "posts", :action => "show", :id =>
    #we're over in the comments controller - how do we get our data
    #errors from the comments model back over to the show page of posts?
    render :controller => "posts", :action => "show" #???

How about "redirect_to :back" ?
That sends you back to the referer.

render :action => 'posts/show' ? I'm not sure if that works though.
I'd actually create create a comments/new page for this purpose.

render :action => 'new'